In order to store digital electronic data it is known to use magnetic tape data cartridges, which are inserted into a tape drive unit having a plurality of read/write heads. Typically, such magnetic tape storage devices may be used to back up data generated by a host device, eg a computer.
In the conventional tape drive, electronic circuitry is provided to encrypt the digital data to be stored, using an algorithm which applies redundancy encryption to the original digital data, so that the data recorded onto the magnetic tape incorporates redundant data from which the original data can be recovered if there is corruption of the data recorded on tape. Such corruption may occur for a variety of reasons, for example, at the edges of the tape due to non-uniform coating of the tape with magnetic material, or due to variations in alignment of the tape with the read/write head. In such cases, on reading the data, areas of corruption are detectable because the data is absent or a usable data signal is indistinguishable in a read signal produced by a read head.
However, another situation where corruption of data can arise and which requires the use of encryption for its correction is where a tape is overwritten and old pre-recorded data from a previous write operation has not been completely erased. This can occur if the tape momentarily loses contact with the read/write head and the new data has not been written over the old. This condition is known as a "data drop out". It is possible that, when the data is subsequently read from tape, old incorrect data could be returned to the host. Since the old data read is present, and may be uncorrupted, it may be mistaken for the correct data which was intended to overwrite the old pre-recorded data.
As the density of data stored on tapes increases and the physical size of tape tracks decreases then the need to correct data dropout defects becomes more important, because increasingly larger amounts of pre-recorded data may fail to be over-written.
A further problem in linear tape data storage devices is identifying where a set of recorded data ends on the tape. Where a new set of data has been overwritten over old pre-recorded data, over a length of tape, the old pre-recorded data, which was written in a same general format as a set of new data overwriting the pre-recorded data, needs to be distinguished from the new data, in order to avoid reading the old data, and mistaking it for new data.